Distribution of load on a ring final for Voltage drop calcs?

[brman]

Just pondering my voltage drop calcs for a fused spur off a ring final. Additional circuit will be around 15-20m long so I though I should do this properly....

My problem is calculating the voltage drop in the existing ring to the point of the spur which of course depends on distribution of load round the ring. I've seen two methods used-
1) 20A at the furthest point and 6A distributed evenly around both sides of the ring. Giving the equivalent of 13A average either side or 26A at the furthest point.
2) 32A distributed evening around the ring. Giving an average or 8A each side or 16A at the furthest point.

I did look at the ESC essential guide on this but it just shows how to calculated it if the actual distribution is known which I can't see working with a socket ring where future use is not known.

What would you guys use?

 

[malcolmsanford]

To find your volt drop on a ring final first divide the length of the ring by 4, you can find the length by R1 + R2

You then use 26amps as a factor because it assumes 20amp at it's furthest point and then 12 distributed around the ring so you have (PD(32) + 20) / 2 = 26

So your formula is

Length/4 X 18 ( VD/a/m for 2.5) X 26 / 1000 and then if you really want to go the whole hog which you should multiply the result by 0.923 for temperature correction

 

[brman]

Thanks Malcolm,

I thought it was the 26A but I couldn't find it in any of my documentation, just on the web somewhere

When you say "find the length by R1+R2" do you mean measure R1+R2 then work out the length using the R1+R2 resistance per meter? (from the OSG)

I was going to use (R1+R2) X (1.5 + 1.5) / (1.5 + 2.5) X 26 which will give the same answer I think. (assuming 2.5 / 1.5 cable)

 

[malcolmsanford]

I didn't realize that I was going to get a test as well lol

Yes to find your length you only really need r1 to be honest, so for an example

r1 @ 0.88/7.41 x 1000 = 118.7m where the 0.88 is the measured value and 7.41 is the metre/mOhm/m value from the OSG

 

[brman]

:lol:

Not meant to be a test, I was just making sure I understood you

Ta.